[PATCH] notifier: Make atomic_notifiers use raw_spinlock

[Sebastian Andrzej Siewior]

From: Valentin Schneider <valentin.schneider@xxxxxxx>
Date: Sun, 22 Nov 2020 20:19:04 +0000

Booting a recent PREEMPT_RT kernel (v5.10-rc3-rt7-rebase) on my arm64 Juno
leads to the idle task blocking on an RT sleeping spinlock down some
notifier path:

|  BUG: scheduling while atomic: swapper/5/0/0x00000002
…
|  atomic_notifier_call_chain_robust (kernel/notifier.c:71 kernel/notifier.c:118 kernel/notifier.c:186)
|  cpu_pm_enter (kernel/cpu_pm.c:39 kernel/cpu_pm.c:93)
|  psci_enter_idle_state (drivers/cpuidle/cpuidle-psci.c:52 drivers/cpuidle/cpuidle-psci.c:129)
|  cpuidle_enter_state (drivers/cpuidle/cpuidle.c:238)
|  cpuidle_enter (drivers/cpuidle/cpuidle.c:353)
|  do_idle (kernel/sched/idle.c:132 kernel/sched/idle.c:213 kernel/sched/idle.c:273)
|  cpu_startup_entry (kernel/sched/idle.c:368 (discriminator 1))
|  secondary_start_kernel (arch/arm64/kernel/smp.c:273)

Two points worth noting:

1) That this is conceptually the same issue as pointed out in:
   313c8c16ee62 ("PM / CPU: replace raw_notifier with atomic_notifier")
2) Only the _robust() variant of atomic_notifier callchains suffer from
   this

AFAICT only the cpu_pm_notifier_chain really needs to be changed, but
singling it out would mean introducing a new (truly) non-blocking API. At
the same time, callers that are fine with any blocking within the call
chain should use blocking notifiers, so patching up all atomic_notifier's
doesn't seem *too* crazy to me.

Fixes: 70d932985757 ("notifier: Fix broken error handling pattern")
Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20201122201904.30940-1-valentin.schneider@xxxxxxx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---

What do we do with this?
Do we merge this as-is, add another "robust atomic notifier" using only
raw_spinlock_t for registration and notification (for only
cpu_pm_notifier_chain) instead of switching to raw_spinlock_t for all
atomic notifier in -tree? 

 include/linux/notifier.h |  6 +++---
 kernel/notifier.c        | 12 ++++++------
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index 2fb373a5c1ede..723bc2df63882 100644
--- a/include/linux/notifier.h
+++ b/include/linux/notifier.h
@@ -58,7 +58,7 @@ struct notifier_block {
 };
 
 struct atomic_notifier_head {
-	spinlock_t lock;
+	raw_spinlock_t lock;
 	struct notifier_block __rcu *head;
 };
 
@@ -78,7 +78,7 @@ struct srcu_notifier_head {
 };
 
 #define ATOMIC_INIT_NOTIFIER_HEAD(name) do {	\
-		spin_lock_init(&(name)->lock);	\
+		raw_spin_lock_init(&(name)->lock);	\
 		(name)->head = NULL;		\
 	} while (0)
 #define BLOCKING_INIT_NOTIFIER_HEAD(name) do {	\
@@ -95,7 +95,7 @@ extern void srcu_init_notifier_head(struct srcu_notifier_head *nh);
 		cleanup_srcu_struct(&(name)->srcu);
 
 #define ATOMIC_NOTIFIER_INIT(name) {				\
-		.lock = __SPIN_LOCK_UNLOCKED(name.lock),	\
+		.lock = __RAW_SPIN_LOCK_UNLOCKED(name.lock),	\
 		.head = NULL }
 #define BLOCKING_NOTIFIER_INIT(name) {				\
 		.rwsem = __RWSEM_INITIALIZER((name).rwsem),	\
diff --git a/kernel/notifier.c b/kernel/notifier.c
index 1b019cbca594a..c20782f076432 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -142,9 +142,9 @@ int atomic_notifier_chain_register(struct atomic_notifier_head *nh,
 	unsigned long flags;
 	int ret;
 
-	spin_lock_irqsave(&nh->lock, flags);
+	raw_spin_lock_irqsave(&nh->lock, flags);
 	ret = notifier_chain_register(&nh->head, n);
-	spin_unlock_irqrestore(&nh->lock, flags);
+	raw_spin_unlock_irqrestore(&nh->lock, flags);
 	return ret;
 }
 EXPORT_SYMBOL_GPL(atomic_notifier_chain_register);
@@ -164,9 +164,9 @@ int atomic_notifier_chain_unregister(struct atomic_notifier_head *nh,
 	unsigned long flags;
 	int ret;
 
-	spin_lock_irqsave(&nh->lock, flags);
+	raw_spin_lock_irqsave(&nh->lock, flags);
 	ret = notifier_chain_unregister(&nh->head, n);
-	spin_unlock_irqrestore(&nh->lock, flags);
+	raw_spin_unlock_irqrestore(&nh->lock, flags);
 	synchronize_rcu();
 	return ret;
 }
@@ -182,9 +182,9 @@ int atomic_notifier_call_chain_robust(struct atomic_notifier_head *nh,
 	 * Musn't use RCU; because then the notifier list can
 	 * change between the up and down traversal.
 	 */
-	spin_lock_irqsave(&nh->lock, flags);
+	raw_spin_lock_irqsave(&nh->lock, flags);
 	ret = notifier_call_chain_robust(&nh->head, val_up, val_down, v);
-	spin_unlock_irqrestore(&nh->lock, flags);
+	raw_spin_unlock_irqrestore(&nh->lock, flags);
 
 	return ret;
 }
-- 
2.32.0